1. Field of the Invention
The present invention relates to a method for the continuous production of thin silicone films, where the film thickness is between 0.1 and 200 μm and the thickness precision is ±5% measured over an area of 200 cm2, and also to the silicone films produced by this method, and to their use.
2. Description of the Related Art
EP0097313A1 describes the production of a thin silicone elastomer film having a film thickness of one to 500 μm. For production, the film is applied to a coated paper, using a coating technique of rollercoating, air doctor or reverse roll coating, wire-wound bar coating, or spray coating in order to produce thin films. These techniques lead to silicone films having unsatisfactory thickness precision, and are therefore of only limited usefulness.
DE2408659 describes a method for producing thin, flawless silicone films, in which the silicone film is produced by casting a centrifuged silicone material to which solvent has been added. The costly and inconvenient method is unsuitable, however, for industrial production in a continuous process. Moreover, the thickness precisions achieved by means of the casting processes described are again insufficient.
EP1661686A2 describes the extrusion or coextrusion of a silicone layer. In terms of thickness precision, extrusion processes are unsuitable for layer thicknesses below about 200 μm, this constituting a substantial disadvantage of this technology. Through the use of a carrier material during the extrusion, moreover, the fluctuations in thickness of the carrier are transferred to the silicone film.
A continuous method for producing thin silicone films is described in DE2045558. The principal disadvantages of that method are that the doctor blade coating, on the one hand, requires high solvent content (ideally 60 wt % solvent is described) and, on the other hand, the accuracy of the layer thickness, analogously to extrusion methods, is dependent on the precision of the carrier used.
As the prior art shows, silicone films in a thickness range starting from about 200 μm and up to several mm can be produced by extrusion methods or molding methods. The respective methods here exhibit different disadvantages. In the case of discontinuous molding methods, a reduction in the layer thickness is accompanied by problems during automatic demolding, and, moreover, the production costs per square meter (sq. m.) of film are higher than in the case of continuously operating processes. Extrusion methods of HTV rubbers have the disadvantage of relatively large variations in thickness, and also of the limited possibility, owing to plant design and slot swelling, of reducing the layer thicknesses below 200 μm.
In addition to the absolute layer thickness, the uniformity of the layer thickness over the whole of the web produced is a critical factor for applications of the film as, for example, a dielectric or a membrane. The requirement of very uniform films having thicknesses of between one and about 250 μm exists in particular in the development of dielectric electroactive polymers (EAP for short), where the film is used as a dielectric. Applications of the electroactive polymers in actuators, sensors, or generators are based on the conversion of electrical energy into mechanical energy or of mechanical energy into electrical energy. Following application of an electrically conducting coating to one or both sides of the film, a deformation in the system as a whole is brought about, for example, by the application of electrical voltage (actuator principle).